Recently, the amount of information dealt with electronic instruments such as personal computers increased dramatically along with the dawn of information-oriented society and electric power consumption of such instruments has markedly increased. Specifically in portable electronic instruments, this increase of electric power consumption has become a major problem for an increased processing capability. Presently in such portable electronic instruments, a lithium battery is generally used as the power supply but energy density of lithium batteries has been pushed to its limit.
In such situations, instead of lithium batteries, the period of continuous use of a portable electronic instrument is expected to be greatly enhanced by the use of fuel cells exhibiting a higher energy density and an enhanced heat exchange rate as a power supply for electronic instruments.
Fuel cells include a solid polymer fuel cell, a phosphoric acid fuel cell, a fused carbonate fuel cell, a solid oxide fuel cell and the like. Of these, the solid polymer fuel cell has advantages of a low operating temperature and a solid electrolyte (thin polymer film). Solid polymer fuel cells are classified mainly into a conversion type polymer fuel cell in which methanol is converted to hydrogen by a converter and a direct methanol polymer fuel cell (DMFC) in which methanol is directly used without using a converter. The DMFC, which needs no converter and can achieve downsizing and weight reduction, is expected to be practically used as a cell used for personal digital assistance (PDA) and the like and also as a dedicated battery toward the coming of ubiquitous society.
The DMFC uses a proton conductive solid polymer membrane as an electrolyte membrane, and generally has a structure in which an anode which is formed of a catalyst coated on porous carbon paper as a diffusion layer and cathode are connected through the electrolyte membrane and an anode-side separator having a channel to supply aqueous methanol solution as a fuel is provided on the anode side, and further, a cathode-side separator having a channel to supply air as an oxidant gas is provided on the cathode side.
When an aqueous methanol solution is supplied to the anode and air is supplied to the cathode, the anode liberates hydrogen ions and electrons concurrently with formation of carbon dioxide gas through the oxidation reaction of methanol and water (CH3OH+H2O2→CO+6H++6e) and the cathode forms water through a reduction reaction of air with the foregoing hydrogen ions which have passed through an electrolyte membrane [6H+(3/2)O2+6e−→3H2O], whereby electric energy can be obtained in an external circuit connecting the anode and the cathode. Therefore, the overall reaction of DMFC is the reaction of methanol with oxygen to form water and carbon dioxide.
In general, an anode or a cathode is formed of a metal catalyst such as platinum, conductive carbon such as carbon black or a catalyst bearing carbon, and a polymer electrolyte. The cost of a platinum catalyst used in an electrode for fuel cells accounts for some tens of % of the total cost, so that reduction of the content of a platinum catalyst is required to reduce cost of such fuel cells.
In conventional methods of preparing a catalyst layer, only 20 to 30% of platinum used in the catalyst layer participates in the electrode reaction, which remains as a problem to be solved for practical use. One reason for this problem is that the catalyst surface causing the decomposition reaction is not close enough to the migration route of produced protons. The migration path is carried by a material containing a proton-accepting group, so that allowing such a material to exist selectively near the catalyst is expected to bring about enhance utilization efficiency of the catalyst.
Proton conduction after reaction takes place only at the three-phase interface in contact with a polymer electrolyte. A fuel is supplied to the three-phase interface, and after reaction, carbon conducts electrons and the polymer electrolyte conducts protons. In conventional methods of preparing a catalyst layer, however, the proportion of a polymer electrolyte existing around the platinum catalyst is relatively small, which renders it difficult to achieve prompt material transfer.
To enhance utilization efficiency of the catalyst, there is disclosed a technique in which a core/shell type catalyst metal comprised of a core of a catalyst metal covered with a catalyst metal different from the core is used and at least 50% by mass of the total catalyst metal amount is accounted for by a catalyst metal carried on the surface of carbon particles in contact with the proton conduction route of a polymer electrolyte (as described in, for example, Patent document 1); a technique of using a carbon containing an organic group capable of dissociating hydrogen ions, such as a sulfonic acid group (as described in, for example, Patent document 2); and an electrode catalyst bearing a catalyst in a carbon material containing at least one ionic functional group on the surface of primary particles of carbon black (as described in, for example, Patent document 3).
There is also disclosed a technique in which to bring a polymer catalyst into sufficient and homogeneous contact with a catalyst to increase the internal reaction area of an electrode, the molecular length of a hydrogen ion-conductive polymer electrolyte is limited to 30-200 nm and when mixing such a polymer electrolyte with a catalyst-bearing carbon through a solvent, it is essential to choose a solvent exhibiting an appropriate dielectric constant (as described in, for example, Patent document 4).
There is also disclosed a technique in which a molecule containing an ion-conductive functional group, capable of functioning as an electrolyte is chemically bonded onto the surface chosen from catalyst particles, other particles and a porous membrane (as disclosed in, for example, Patent document 5).
There is also disclosed a grafted platinum-bearing catalyst as a technique to enhance the utilization efficiency of a platinum catalyst, in which a monomer is allowed to react on the carbon surface and chemically bonded thereto, whereby an electrolyte polymer is fixed on the carbon surface (as described in, for example, Literature 1).
However, the foregoing techniques do not always produce a proton migration route close enough to the vicinity of a catalyst, resulting in little gain in enhancement of efficiency. Accordingly, there is desired development of an electrode exhibiting an enhanced catalyst utilization efficiency in which a proton conduction route is effectively prepared.
Patent document 1: JP-A No. 2001-118582 (hereinafter, the term JP-A refers to Japanese Patent Application Publication)
Patent document 2: JP-A No. 2004-7 Patent document 3: JP-A No. 2004-2 Patent document 4: JP-A No. 2002-6 Patent document 5: JP-A No. 2004-17 Literature 1: Jisedai Nenryodenchi no Gijutsuhokokukai Youshishu (Heisei 15. 12. 14) Dokuritsu Gyouseihojin Sangyogijutsu Sogoukaihatsukikou Nenryodenchi•Suiso Gijutsu Kaihatsubu